1. Field of the Invention
The present invention relates to controlling a legged robot so that the robot does not fall. More particularly, the present invention relates to controlling the robot based on the rate of change of the robot's angular momentum.
2. Description of Background Art
In order to engage in useful activities, a legged robot should be able to maintain its balance. The term “balance” generally refers to the preservation of overall rotational stability or equilibrium. If a robot's overall rotational equilibrium is lost, the robot can fall.
Control systems have been developed that instruct robots to take certain actions in an attempt to maintain or improve the robots' balance. A control system is usually based on a “stability criterion,” which is a physical quantity that represents a robot's rotational equilibrium (or lack thereof). The value of the stability criterion varies based on the robot's state, and there is usually a range of values that indicate stability. If a robot's stability criterion does not fall within this goal range, the control system instructs the robot to take certain actions. These actions are meant to change the robot's state such that the value of the stability criterion approaches (or reaches) the goal range. By continuously monitoring the stability criterion, the controller can cause the robot to maintain or improve its stability over time.
One type of stability criterion is a point on the support surface (e.g., the ground) on which the robot is standing. One such point is the center of pressure (CoP). The CoP is the point of application of the resultant ground reaction force (GRF) underneath the robot's feet. Thus, the CoP exists as long as the robot is not airborne, since at least one of its feet is touching the ground at all times. The location of the CoP can be experimentally measured. In order for the robot to be stable, the CoP should be located within the robot's support polygon and, ideally, at the most central location within the polygon.
In robotics literature, the CoP is sometimes referred to as the zero moment point (ZMP). This literature provides a means to analytically compute (rather than experimentally measure) the location of the CoP/ZMP. The location of the CoP/ZMP is not well-defined when the support surface is non-planar. Although the location of the CoP/ZMP can quantify the stability of a relatively stable robot, it cannot do so for an unstable robot. This is because, in certain cases, the same CoP/ZMP location can correspond to several different states of a robot, and these states can have varying effects on the robot's stability.
Another such point is the foot-rotation indicator (FRI) point. The FRI point is related to the phenomenon of foot rotation and is applicable only during the single support phase of a biped. While, by definition, the CoP cannot leave the support polygon, the FRI point can. When the FRI point is located outside the support polygon, the distance between the FRI point and the support polygon is proportional to the amount of instability. The location of the FRI point is undefined when more than one foot is touching the ground.
What is needed are a stability criterion that overcomes the disadvantages of the previous criteria and a control technique that uses the criterion to maintain or improve a robot's balance.